41. First Missing Positive（找到数组中未出现的最小正整数）

Total Accepted: 87074
Total Submissions: 348588
Difficulty: Hard
Contributors: Admin
Given an unsorted integer array, find the first missing positive integer.

For example,
Given [1,2,0] return 3,
and [3,4,-1,1] return 2.

Your algorithm should run in O(n) time and uses constant space.

方法：
遍历一遍，把正确合法的正整数方在正确的位置上即可。比如我们找到元素5，就把5换到A[4]上，因为数组从0下标开始。最终结果数组A[i] != i+1 的就是该位置上的正整数没有出现过，返回i+1即可。

代码如下：

class Solution(object):
    def firstMissingPositive(self, nums):
        n = len(nums)
        for i in range(0, n):
            while nums[i] > 0 and nums[i] <= n and nums[nums[i]-1] != nums[i]:
                nums[nums[i]-1], nums[i] = nums[i], nums[nums[i]-1]
        for i in range(0, n):
            if nums[i] != i+1:
                return i+1
        return n+1

呵呵，人生苦短，我用Python。